Several factors combine to make some volcanic eruptions a danger to aircraft. The first is that the particles in volcanic ash are composed of hard materials, such as rock, glass and sand. The second factor is that when these particles are ejected high enough, they may be picked up by the high winds that can disperse them over large distances. A third factor is that the dust plume is not dense enough to be easily seen by airline pilots, but it is dense enough to cause severe damage to the engines, which may cut out and fail. Failure happens when the fine dust particles block up the air vents.